In the processing of plastic materials, the dehumidification treatment of plastic material granules performed before fusion, is of particular importance.
As is known, on account of their hygroscopicity plastic materials in granular form contain molecules of water. During the fusion step the molecules of water may insinuate themselves into the molecular chains of the polymers and break them. This causes surface defects in the final products, bubbles and lack of structural and color homogeneity to the detriment of the quality of the final product.
The plastic materials in granules to be subjected to dehumidification are typically stored in hoppers connected in a watertight manner to one or more hot and dry air generator devices, in the jargon called “dryers”, which blow hot, dry air into the hopper.
Once it has entered the hopper, the dry air (hereinafter called “process air”) traverses the mass of plastic material in granules to be dehumidified entirely or in part, removing the humidity contained in them and leaving the hopper through a dedicated exit duct.
As is known, achieving the desired degree of dehumidification for a given granular plastic material which will later be subjected to fusion in a transformer machine, depends on many factors, in particular the residence time of the granular material inside the dehumidification hopper, the specific flow of process air (hereinafter also referred to as “specific airflow”) sent to the hopper, the processing temperature and thermo-fluid-dynamic behavior associated with the interaction between the current of process air and the granular plastic material, dependent on the geometry of the hopper.
The residence time τ is understood to refer to the ratio between the quantity of material inside the hopper and the hourly production imposed by the transformer machine.
Depending on the degree of dehumidification required for a given granular material to be treated, the granules of material must remain inside the hopper for a certain, specific interval of time, at a certain process temperature, such as to permit the diffusion of the molecules of water from the inside of the granules to the outside.
As is known accepted practice requires that the residence time of the granular material to be treated be set a priori on the basis of tabular values in literature, variable according to the type of material. The volume of the hopper is calculated on the basis of the presumed residence time and the hourly production of material to be treated (imposed by the transformer machine which the hopper must serve). From this it derives that the residence time cannot be considered a variable of the system but is rather a prefixed parameter.
To modify the degree of residual humidity which the granular material presents at the end of the dehumidification treatment, the characteristics of the process air are acted on, modifying the specific airflow, the temperature and/or the dew point temperature.
In such regard it is to be noted that the dew point temperature is defined as the temperature at which, at constant pressure, the air (or more specifically the air-vapor mixture) becomes saturated with water vapor. The dew point temperature is obviously related to the relative humidity of the airflow.
Traditionally the dehumidification process is therefore managed on the basis of standard conditions presented in literature for the different types of material to be treated.
On account of the variability of the conditions in which the dehumidification process takes place inside the hopper (for example related to the thermo-fluid dynamics inside the hopper) dehumidification may not prove entirely satisfactory, with a degree of residual humidity in the treated material over the acceptable maximum limit.
Generally, to overcome this problem the dehumidifying capacity of the process air tends to be prudentially increased compared to the standard conditions envisaged, increasing the specific flow and the temperature and/or decreasing the humidity content of the process air (i.e. lowering the dew point temperature). This obviously has as a consequence a reduction of the energy efficiency of the process.
Solutions of plants have been proposed which envisage regulating the system variables on the basis of the degree of residual humidity detected in the material leaving the hopper, after the dehumidifying treatment, as taught in the US patent application published as no. US2007/0277392.
More in detail, a detection sensor of the humidity of the granular material is positioned at the output mouth of the hopper, so as to regulate in feedback the process variables.
This system however, while improving control of the dehumidification process, is still not entirely satisfactory. The regulation proposed in fact does not have an instantaneous benefit on the quality of the material treated. The material leaving the hopper may therefore not fall within the tolerance limits required.
Moreover, it is widely subject to instability phenomena. It may in fact happen that on account of the variability of the conditions in which the process takes place, due both to the characteristics of the granular material treated and to the thermo-fluid dynamics, the adjustments of the parameters imposed by the system prove opposite to those effectively needed.